Parallel evolution despite low genetic diversity in three-spined sticklebacks

The three-spined stickleback (Gasterosteus aculeatus) is a model organism for studies of parallel evolution in the wild; marine stickleback populations have repeatedly colonized and adapted to different brackish and freshwater habitats. Population genetic studies of European three-spined sticklebacks have usually been conducted only in high-latitude areas. Here, we analysed southern and northern European samples of marine and freshwater three-spined stickleback to test two hypotheses. First, southern European freshwater populations – which currently lack or have limited connection to marine populations – have lost genetic diversity due to population bottlenecks and inbreeding compared to their northern European counterparts. Second, the degree of genetic parallelism in response to freshwater colonisation is higher among northern than southern European populations as the latter have been isolated and likely subjected to strong genetic drift. The results show that southern populations exhibit lower genetic diversity but a higher degree of genetic parallelism than northern populations. Hence, they confirm the hypothesis that southern populations have lost genetic diversity, but this loss likely happened after they had already adapted to freshwater conditions, explaining the high degree of genetic parallelism in the south.